The present invention generally relates to fluid pressure relief valves of the type operated by a pilot valve and, more particularly, to pilot operated relief valves of the non-flowing type.
A safety pressure-relief system typically includes a relief or main valve mounted over a pressure vessel, such as a tank or flow line, and a pilot valve which controls operation of the main valve. The main valve moves to an open position when the fluid pressure within the vessel rises above a predetermined set value to relieve or reduce the fluid pressure within the vessel and moves back to a closed position when the fluid pressure within the vessel is reduced below the set value.
Preferably, the pilot valve is of a non-flowing type wherein no system fluid flows through the pilot valve when the main valve is in a static condition below the set value or open and flowing. Non-flowing pilot valves typically have a higher sensitivity and a faster response to pressure changes in the fluid as compared with pilot valves having a continuous fluid flow therethrough. Additionally, these pilot valves may have superior operation when the relief valve is returned to its closed position because the pilot valve is not influenced by flowing system fluid.
Typically, the fast response of these pilot valves is obtained by a snap or pop action of valve elements. For example, see U.S. Pat. No. 3,664,362, disclosing a non-flowing type pilot valve, the disclosure of which is expressly incorporated herein by reference in its entirety. Fluid flows through the pilot valve and changes a dome pressure of the main valve to activate a fluid pressure responsive element, such as a piston or diaphragm. The valve elements are quickly snapped to a full open condition.
This fast response, however, may at times need to be limited. For example, pressure spikes or extremely short duration rises in pressure may cause system fluid to be discarded due to the fast reaction of the pop-action valve elements. Thus, the fast reaction may be too fast and result in an excessive loss of system fluid, that is, a greater volume of system fluid than is required to reduce the system pressure back to the set value. This excessive loss of system fluid is particularly a concern when a relatively expensive system fluid is discarded.
There have been prior attempts to solve this problem by forming non-flowing pilot valves with modulating action, that is, the relief valve is partially opened an amount in proportion to the magnitude of the overpressure. For example, see U.S. Pat. No. 4,586,533, disclosing a non-flowing type pilot valve, the disclosure of which is expressly incorporated herein by reference in its entirety. While these pilot valves have provided some modulating action, the amount the relief valve opens is still not proportional to the magnitude of the overpressure in a substantially linear manner. Accordingly, there is a need in the art for a non-flowing pilot operated relief valve which provides improved modulating action.